Cumulonimbus Clusters in Warm Air are mesoscale α/β cloud phenomena with an average diameter of 200 km. Over land they start to be generated when the temperature of the surface is sufficient to trigger convection. They dissolve during the evening, in contrast to MCSs which can exist during the night and morning. The highest frequency is in summertime over land, but they can also appear over the sea during the whole year, at any time during day or night. CB Clusters in Warm Air do not arise at the boundary of two air masses like Cold or Warm Fronts, but develop ahead of the Cold Front in the area of a thermal ridge. CB Clusters often are embedded in cellular cloudiness with lower cloud In contrast to MCS single CBs are usually well separated from each other allowing easy detection in radar images.
The synoptic environment shows great resemblance with that of an MCS (see Cumulonimbus (Cb) and Mesoscale Convective System (MCS) - Meteorological Physical Background ). Often a CB Cluster is a preliminary stage in the life cycle of an MCS.

Life cycle of a CB Cluster in Warm Air

At 18.00 UTC at the western part of the Pyrenees the initial stage of a new CB Cluster is developing into a MCS.

As seen in the above images the two CB Clusters did not develop into an MCS, but in the same airmass a Cb Cluster closer to the Cold Front followed the whole life cycle from CB Cluster into an MCS. These complete life cycles are often observed close to the Cold Front where cold air advection at higher levels plays an important role.

A CB-cluster developing into an MCS

Instability

The favourable area for the development of CB Clusters is characterised by a (potentially) unstable humid airmass.

In most cases the relative humidity values in the lower layers are high, indicating high values of precipitable water. In the investigated cases an average of about 30mm was found, with a dispersion of 10mm.

The average height of the potentially unstable layer is characterised by the inversion of the ThetaW. In the investigated cases a common height of this inversion was 700 hPa.

CB Clusters are situated in an area where the instability index is highest, CBs develop rapidly and in more than 90% of the cases thunderstorms are reported. Vertical windshear in speed and direction is necessary for the development of thunderstorms. (see Cumulonimbus (Cb) and Mesoscale Convective System (MCS) - Meteorological Physical Background , the Role of vertical wind shear). In the description of this conceptual model the Boyden Index is used.

Pre-frontal thickness ridges

As in the case of MCSs, CB clusters in Warm Air occur in the presence of a thickness ridge (see Cumulonimbus (Cb) and Mesoscale Convective System (MCS) - Meteorological Physical Background ) and a ridge in the wet bulb potential temperature (ThetaW).

Convergence

Cbs and thunderstorms in a warm air mass develop because of the heating of the surface in an unstable environment. Dynamical forcing by Positive Vorticity Advection (PVA) at higher levels plays a minor role. Convergence in the lower levels, however, can accelerate the development, especially in moist areas.
Differential heating of the earth surface causes the formation of troughs in the surface pressure field which leads to convergence in the lower levels. Orographic effects in mountaineous terrain also play an important role in this convergence mechanism.

Temperature advection

Temperature advection is not normally so important for the developmenbt of CB Clusters, see the low value in the diagram, although in most cases some cold advection at higher levels can contribute to the triggering of CBs, especially during the preliminary stage of an MCS. In such cases a Warm Conveyor Belt in the lower levels in front of the Cold Front also plays a role in destabilizing the atmosphere by its warm advection in the lower levels.